Make standard use a proper version of sphinxcontrib-jsonschema

[siwhitehouse]

Currently the data standard is using a forked version that has not been released properly.

We must make sure it's using a properly released version, otherwise there will be problems getting updates and fixes in the future.

Copied across from an (internal) issue logged by @odscjames

[siwhitehouse]

@odscjames I've assigned this to you as you raised it initially.

[odscjames]

By coincidence, just yesterday I looked at this issue. Planio 23431

[siwhitehouse]

That is a coincidence. I'll leave you to move across the relevant bits of the Planio issue when you're ready. Thanks James.